The present invention generally relates to fuel injection systems for Diesel engines and, more particularly, to a fuel injection system of the type which includes a booster for boosting the pressure of fuel to be supplied to a fuel injector and a nozzle needle actuator for controlling a fuel injection by the fuel injector in response to a control of a hydraulic fluid pressure applied thereto.
A prior art fuel injection system of the type described includes a fuel reservoir, and a booster operated by a pressure differential between opposite ends thereof to compress fuel fed from the fuel reservoir to one end thereof. The fuel develops a first hydraulic fluid pressure. A fuel injector injects a supply of compressed fuel fed from the booster. A nozzle needle actuator is operatively associated with the fuel injector and operated by a pressure differential between opposite ends thereof to start and terminate a fuel injection from the fuel injector. The supply of compressed fuel from the booster is also fed to one end of the nozzle needle actuator to develop the first hydraulic fluid pressure. A first hydraulic circuit means produces a variable hydraulic fluid pressure and is communicated with a hydraulic fluid reservoir. The variable hydraulic fluid pressure is fed to the other end of the booster through a first direction control means as a second hydraulic fluid pressure. The other end of the nozzle needle actuator is communicated by a second hydraulic circuit means to the fluid reservoir and the first hydraulic circuit means or second hydraulic fluid pressure through a second direction control means. A control means controls the second hydraulic fluid pressure in the first hydraulic circuit means and the states of the first and second direction control means.
This type of fuel injection system, however, involves a problem due to the use of a solenoid operated direction control valve as the second direction control means which selectively communicates said other end of the booster to the fluid reservoir and a pump associated therewith. The solenoid operated direction control valve is of the ordinary type in which a spool disposed in a valve body is caused into a stroke to switch the flow passage from one to the other. The maximum switching rate available with such a valve is not more than five times per second and the buildup characteristic is poor. Therefore, the prior art system cannot speed up its operation beyond a limit determined by the valve.
Another inherent drawback of the spool type valve is that the structure is not suitable for accommodating a large flow rate of fluid.